1. Technical Field
The present invention relates to a method of and apparatus for the serving of computer files. It has application, in particular, to the secure serving of digitally signed computer files.
2. Related Art
The notion of associating a sign of some form with a document or an object to denote, for example, authorship or ownership has long been in existence. It is an unfortunate reflection on human nature that the related notion of falsely associating a sign with a document or with an object to indicate false authorship or ownership has also long been in existence.
With the advent of the printing press and the printed document and, more recently, the digital or electronic computer and the digital or electronic document, the problems of the faithful reproduction and the convenient editing or alteration of documents have been much ameliorated.
As will be well known, a digital document can typically be altered or copied as many times as is wished without any change in quality since it is only the digital bits representing the information content of the document that are changing. If a digital document is created by a first party and then covertly altered by a second party, it may well be difficult for a third party subsequently reading the document to tell that it has been altered.
The advent of networked communication between computers and in particular the rise of the Internet and the World Wide Web has meant that vast numbers of computers all over the world can now communicate with each other using common protocols. Electronic documents are often now made available as Web Pages on a Web Site.
It will be well known that the World Wide Web (or simply ‘Web’ hereinafter) has a wide variety of associated concepts and standards. A rich source of information relating to these concepts and standards is the World Wide Web Consortium (http://www.w3c.org), a body hosted by the Laboratory for Computer Science at the Massachusetts Institute of Technology (MIT). Concepts such as a ‘Web Server’, a ‘Web Site’, a ‘Web Page’, a ‘Web Browser’, a ‘Hyperlink’ and a ‘Uniform Resource Locator’ and standards such as the ‘HyperText Transport Protocol (HTTP)’ and the ‘HyperText Markup Language (HTML)’ will be well known.
A problem faced by those parties wishing to distribute content in the form of electronic documents or files, for example, on the World Wide Web, has been the vulnerability of the stored content to deliberate alteration by unauthorised third parties accessing the content over a communications network.
Should an unauthorised third party manage to access a given Web Server, they might, for example, edit a Web Page stored on that Web Server. When the Web Page is subsequently viewed with a Web Browser, the content of the Web Page would then reflect the message of the unauthorised third party rather than the original content provider.
It will be appreciated that a wide variety of motives may exist for unauthorised third parties to attempt to subvert the message delivered by a given piece of content but it is probably safe to assume that in all cases the content provider would prefer not to have the message delivered by its content tampered with and then presented to the browsing world as its own.
A first present day approach to tackling this problem of the vulnerability of stored content to alteration by unauthorised third parties might attempt to ensure that the stored content is never accessible to unauthorised third parties.
One example of this approach is the use of a so-called ‘firewall’. As will be well known a firewall may be used to protect a computer connected to a network by controlling traffic between the computer and the network such that only certain types of traffic, as defined by the computer administrator, are allowed to pass from the network to the computer or vice versa. In theory this should prevent unauthorised third parties from accessing the computer from the network such that they could alter the content stored on that computer. Naturally such a firewall cannot protect the stored content from alteration by a malicious user validly operating inside the firewall
In practice it will be well known that real-world implementations of firewalls are often far from secure.
A second approach, mindful of the fact that the content might have been altered either when stored or during transmission over a communications channel, is to perform a check on downloaded content to see if it has been tampered with.
One simple example of this second approach is the use of a so-called ‘checksum’. As will be well known a checksum is computed from a given block of data, yielding a value which is then associated with that block of data. If the checksum computation is run again, any change in the data will cause a change in the checksum value. Checksum methods are most often employed as a simple check to detect corruption during transmission of data. In theory then, when a given piece of content is downloaded along with an initial checksum value, a new checksum value can be computed for the downloaded content, which can then be compared with the original checksum value sent along with the content. If the original checksum value and the newly derived checksum value are the same then there may be some confidence that the content has not been altered after the computation of the original checksum value, which might be either whilst stored or during transmission.
In practice it will be appreciated however, that any unauthorised third party able enough, for example, to access and tamper with stored content may well be able enough to alter the original checksum accordingly. If this were done then the checksum comparison performed when the content is downloaded would falsely indicate that the content had not been tampered with since its original storage.
More sophisticated examples of this second approach involve the use of so-called ‘digital signatures’. The theory and practice of digital signatures have become very well known over the past few years as the Internet and more particularly the World Wide Web have experienced exponential growth.
A treatment of digital signatures may, for example, be found in ‘Applied Cryptography: Protocols, Algorithms and Source Code in C’ by Bruce Schneier, second edition 1996, John Wiley & Sons. A further treatment of digital signatures may be found in ‘PGP: Pretty Good Privacy’ by Simson Garfinkel, first edition 1995, O'Reilly & Associates. Terms such as ‘public key’, ‘private key’, ‘hash function’ and ‘message digest function’ will be well understood.
Digital signature techniques utilise so-called ‘public key’ cryptographic methods. As will be well known, public key cryptography uses an algorithmically related pair of keys, a so-called ‘public key’ and a so-called ‘private key’, to encrypt messages, rather than the single key of more traditional symmetric key cryptography. The public key is intended to be widely distributed in the public domain whereas the private key must be kept absolutely secret. Crucially, knowledge of the public key does not allow the private key to be determined. Typically, a message encrypted with a public key is decrypted and can only be decrypted with the corresponding private key. The encryption process is symmetric however such that an encryption operation performed with the private key can be decrypted with the public key. A successful decryption with a given public key guarantees that the message was encrypted with the matched private key.
Public key cryptography can be used to attempt to secure a communications channel such that content transmitted over that channel cannot be intercepted and compromised. One example of such an application is the Secure Sockets Layer (SSL) protocol, originally developed by the Netscape Communications Corporation (Mountain View Calif., USA). For communication between, for example, a client computer and a server computer, such a protocol first authenticates the server computer using public key cryptography and then shares a symmetric key for use in encrypting all further communication between the client and server computers. A protocol such as SSL thus both protects against a first server computer pretending to be a second server computer and serving data falsely purporting to be from that second server computer and prevents any unauthorised third party intercepting and altering communications during transmission. Such a protocol is, however, aimed at the securing of content during transmission, not at solving the problem of the stored content being vulnerable to alteration.
Digital signature techniques using public key cryptography allow checks not only as to ‘authentication’, guaranteeing that a digitally signed ‘document’ does in fact originate from the party whose signature the document bears but also as to ‘integrity’, guaranteeing that the contents of the document have not been tampered with since the originating party digitally signed the document.
The process by which digital signatures are employed in order to perform a check on downloaded content to see if it has been tampered with will be discussed below in greater detail having regard to the invention. It will suffice at this point to consider the functionality provided through the use of digital signatures in the following example of the second approach.
The Microsoft Corporation (Redmond Wash., USA) has developed so-called ‘Authenticode™’. Authenticode™ software is installed on client computers and is directed towards checking software that has been downloaded over a network from, for example, a server computer, to see if the software has been tampered with in an unauthorised fashion. Each such piece of code will have been digitally signed. Having regard to a particular piece of digitally signed code downloaded over a network, before the installation or execution of the code, Authenticode™ may check the digital signature to see if it is valid. A selection of a ‘high’, ‘medium’ or ‘none’ Authenticode™ safety setting must be made in the client software. With a ‘high’ setting Authenticode™ will not allow the installation or execution of code whose associated digital signature proves to be invalid. With, however, a ‘medium’ setting, Authenticode™ will warn the user that the code is ‘untrustworthy’ but will allow the option of installing or executing it if the user wishes. With a safety setting of ‘none’, Authenticode™ provides no such warning.
As will be evident, an arrangement such as Authenticode™, checking code downloaded to a client at that client, can only go so far in protecting stored content. Such checking performed at the client will involve the sending of the content in question to the client computer. In this way, content that has been tampered with will still be sent out over the network to the client. It may be that an arrangement such as Authenticode™ may be configured to deny the installation or execution of an ‘untrustworthy’ piece of code, but the code still exists at the client and it cannot be guaranteed that an able enough user could not access it. Alternatively, it is clear that such a configuration can be changed to allow the installation or execution of ‘untrustworthy’ code if so wished.
It will be appreciated that neither with the first approach to the problem of the content alteration (attempting to ensure that the stored content is never accessible to unauthorised third parties), nor with the second approach, (attempting to perform a check on downloaded content to see if it has been tampered with), is it guaranteed that the altered content will not be seen.
In the first case if, for example, the relevant firewall had been breached and unbeknownst to the Web Site administrator the stored content had been altered, the altered content would be viewed by anyone accessing the Web Page until such time as the Web Site administrator noticed or was informed of the alteration and took corrective action.
In similar fashion, in the second case if, for example, the digital signature authentication of the relevant downloaded content had failed, then, as mentioned above, although the content will be deemed ‘untrustworthy’, it may well be open to the ‘downloader’ to view or otherwise execute the ‘untrustworthy’ altered content. Indeed, a situation can be imagined where the notoriety of a Web Page that had been tampered with by an unauthorised third party is the very reason for persons wanting to view the Web Page, before the Web Site administrator can take corrective action. Again, even if the downloader is prevented from, for example, executing an untrustworthy file, that file has still been sent out over a network and it may well be possible to access a copy of the file at some point in the process.